Absorption refrigerators

ABSTRACT

This invention relates to a pump and refrigeration system including an evaporator from which refrigerant vapor is withdrawn by an absorber and absorbent is recharged by a generator. Further, a condenser is provided between the generator and the evaporator so that refrigerant vapor from the generator can be condensed prior to being returned to the evaporator. In addition, the system is provided with an ejector which is positioned downstream of the evaporator so as to withdraw refrigerant vapor from same and upstream of the condenser so that said withdrawn refrigerant vapor passes through the ejector to the condenser. Thus, in the system of the invention, both an absorber and ejector withdraw refrigerant vapor from the evaporator, thus enhancing the efficiency of the system and further refrigerant vapor passing through the ejector is delivered directly to a condenser, thus reducing the burden of the absorber.

The invention relates to heat pump and refrigeration systems and inparticular to a reversible heat pump and refrigeration system which iscombined with an injector, ejector or jet pump, hereinafter referred toas elector.

The environmental case for using heat operated refrigeration and heatpump cycles instead of vapour compression types is strong. For example,some more complex, ie multiple-effect absorption refrigerators typicallyused in air conditioning applications are reported to have effectivecoefficient of performance (COP) values, (in terms of primary energyconsumption), approaching 1.5, whereas vapour compression systems,powered by mains electricity, seldom have effective COP values greaterthan 0.9 when the inefficiencies of electrical power supply are takeninto account. A comparison of these COP values indicates the potentialfor a 70% reduction in CO₂ emissions is possible by changing over toabsorption refrigerators. This is in addition to the potentialenvironmental benefits of using environmentally friendlier refrigerants,such as water.

Unfortunately, less complex, ie single-effect absorption refrigeratorstend to be less efficient than either of those described above. Forexample, they tend to have a COP in the region of 0.4-0.45. Theirperformance is therefore less than multiple-effect absorptionrefrigerators and vapour compression refrigerators. Moreover, they alsotend to be more costly in terms of capital investment per kW of cooling.

One important application of refrigeration and heat pumping is inbuilding air conditioning. At this time there is an increasing trendaway from at large centralized refrigeration plant for both economic andenvironmental control reasons. This trend is recognized by theincreasing sales success of split, multi-split and Variable RefrigerantVolume (VRV) systems, all of which include small mains powered vapourcompression refrigerators. The vast majority of systems sold havecooling capacities of less than 30 kW. However, at this time, absorptionrefrigerator units are generally only available with cooling capacitiesranging from 300 kW to 6000 kW.

The need for a cost effective and efficient absorption refrigerator inthe small capacity range is recognized. However, the small scalerefrigerator market is particularly price sensitive and verycompetitive. Further research into heat powered refrigerator technologyis required if efficient and cost effective units are to become widelyavailable and the environmental benefits realized.

Our aims for the future development of refrigeration machines mustinclude a cessation to the use of synthetic refrigerant fluids, such asCFC, HCFC and HFC refrigerants, and also significant cuts in CO₂emissions associated with operating refrigeration equipment. One way toachieve these aims is to encourage users of refrigeration equipment toselect heat powered refrigerator options, as opposed to vapourcompression options.

We therefore want to provide a heat pump and refrigeration system whichis adapted so that the load on the absorber in reduced.

It is known to provide heat pump and refrigeration systems which includean ejector, which is arranged so as to be upstream of a condenser. Forexample, U.S. Pat. No. 4,290,273 describes such a system but it is ofnote that the ejector is not used for the purpose of extractingrefrigerant vapour from the evaporator and so reducing the demands ofthe absorber so as to increase the efficiency of the system. On thecontrary, the provision of an ejector has no effect on the load on theabsorber and therefore the relative positioning of the ejector in thesystem described in this patent document is of no relevance to thesubject matter of this invention.

Similarly, U.S. Pat. No. 3,440,832 also described a system incorporatingan ejector, which ejector is position upstream of the condenser.However, this document similarly does not address how to reduce the loadon an absorber but rather it tends to teach away from the inventiondescribed in this application in that it addresses how to minimise theimpact of an extreme load on an absorber.

It is therefore an object of the invention to provide a heat pump andrefrigeration system which is heat powered and therefore environmentallypreferable, and of a small scale, and therefore commercially preferable.

According to the invention there is therefore provided a heat pump andrefrigeration system comprising;

a generator for producing heat to power the system;

a condenser for rejecting heat from the system;

an evaporator for effecting heat exchange with an environment;

an absorber for extracting refrigerant vapour from the evaporator; and

an ejector for extracting refrigerant vapour from said evaporatorcharacterised in that;

the ejector is positioned downstream of said evaporator and upstream ofthe condenser so that refrigerant vapour extracted from said evaporatorby the ejector, passes through the ejector, before being delivereddirectly to the condenser.

In the above arrangement the refrigerant vapour passing through theejector is compressed so facilitating condensation of same in thecondenser.

Moreover, since some of the refrigerant vapour extracted from theevaporator is entrained via the ejector to the condenser, the degree ofprocessing required by the absorber is relatively reduced. This meansthat in a system of the invention, per kW cooling, the size of theabsorber can be reduced so that it is a half to two-thirds less thanthat typically required in a conventional system. Furthermore, the sizeof the condenser remains unchanged. Since the absorber is a relativelycomplex, large and costly component of the system, it will be apparentthat a modification in accordance with the invention, has a number ofadvantages because it reduces the cost of the system and furthermore,reduces the complexity whilst providing for good performance.

In a preferred embodiment of the invention the ejector is alsopositioned downstream of the generator so that fluid, for example vapourrefrigerant such as steam, issuing from the generator and passingthrough the ejector provides a means for entraining vapour refrigerantfrom the evaporator to the ejector.

In this preferred arrangement the fluid issuing from the generator is inthe form of a vapour and those skilled in the art will appreciate thatthis provides for maximum efficiency in the operation of the ejector.

Preferably liquid refrigerant passes from the condenser to theevaporator and then, upon vaporising in the evaporator, the vapourrefrigerant passes to both the ejector and the absorber. It followsthat, in the system of the invention, all of the refrigerant fluidpasses through the evaporator. The significance of this will becomeclear hereinafter with reference to the prior art.

The efficiency of the system, otherwise measured as a ratio betweencooling capacity at the evaporator and the heat input to the generator,will be determined by the amount of refrigerant vapour drawn through theejector from the evaporator plus the refrigerant drawn into theabsorber.

The use of an ejector in a heat-powered refrigeration system orabsorption refrigerator has been described in the prior art but theabove arrangement and corresponding advantages have not hitherto beendisclosed or realized.

For example Kuhienschmidt disclosed in U.S. Pat. No. 3717007 that anabsorption cycle using salt absorbent based working fluid was capable ofoperating at low evaporator temperatures and of employing an air cooledabsorber, without the problem of crystallization. A schematic diagram ofthis cycle is shown in FIG. 5. This cycle consists of double-effectgenerators, however, in contrast to a conventional double-effect system,the low pressure vapour refrigerant from the second-effect generator isused as the primary fluid in an ejector which entrains the refrigerantvapour from the evaporator. This means that none of the refrigerantsfrom the second-effect generator passes through the evaporator. Thus notall of the refrigerant in the system is used for the purpose of heatexchange in the evaporator. This tends to be inefficient.

The ejector exhaust is discharged to the absorber to maintain thepressure differential between the evaporator and the absorber. Thismeans that the absorber must process refrigerant from the first-effectgenerator and so passing through the evaporator, and also refrigerantfrom the second-effect generator which by-passes the evaporator.Consequently, the absorber must process refrigerant which does notdirectly participate in heat exchange within the evaporator. This tendsto be inefficient. Moreover, the more processing the absorber has to do,the greater its size and complexity and, correspondingly, that of thesystem.

It should be noted that there is no condenser in this cycle as the highpressure refrigerant vapour is condensed in the second-effect generatorand the low pressure refrigerant vapour is used as the primary fluid forthe ejector.

Similarly Chen et al disclosed in the Journal of Applied Energy Volume30 Pages 37 to 51, a cycle with an ejector using high temperature liquidsolution returning from the generator as a primary fluid and arefrigeration vapour from the evaporator as a secondary fluid. The useof the liquid as a primary fluid in the ejector is less efficient thanusing vapour derived directly from the generator.

The ejector exhaust is discharged to the absorber as shown in FIG. 6.Again, the absorber is responsible for processing all the refrigerantflowing through the system. Accordingly, the size and complexity of theabsorber must be modified accordingly. Differential pressure ratiosbetween the absorber and the evaporator between 1.1-1.2 are claimed.

Computer simulations of the herein disclosed invented single-effectsystem indicate that COP values approaching those obtainable fromdouble-effect cycles are possible but with less complex construction.Products based on the new design can be both more compact and cheaperthan conventional equipment in terms of price per kilowatt of cooling.The proposed cycle would also be more easily reversible compared withthe double-effect system and can provide higher sink temperatures withsimilar COP values. Further increases in COP may be achieved with theintroduction of an economiser unit into the combined ejector-absorptioncycle.

The most rapid application will be for custom-built equipment, withsubsequent development of mass-market devices both directly, incollaboration with a major partner, and/or through licensing of thetechnology.

An embodiment of the invention will now be described by way of exampleonly with reference to the following Figures wherein

FIG. 1 represents a diagrammatic view of a conventional single-effectabsorption cycle;

FIG. 2 represents a diagrammatic view of it novel ejector-absorptionsystem in accordance with the invention;

FIG. 3 represents a diagrammatic view of a novel ejector-absorptionsystem in accordance with the invention which further includes aseparator;

FIG. 4 represents a novel ejector-absorption system in accordance withthe invention which further includes an ejector economiser;

FIG. 5 is a schematic diagram of the Kuhlenschmidt absorption cycle; and

FIG. 6 shows a conventional system where the ejector exhaust isdischarged to the absorber.

Referring firstly to FIG. 1 there is illustrated a conventionalabsorption heat pump and refrigerator system which in its simplest formcomprises a generator 1 in fluid connection with a condenser 2 which isin turn in fluid connection with an evaporator 3. The evaporator 3 is influid connection with an absorber 4 which ultimately is in fluidconnection with the generator. Thus a system comprising at least fourmembers is illustrated.

For the purpose of description it is assumed that the absorbent and theabsorber is lithium bromide and the refrigerant is water. Refrigerant(water) vapour flows from the evaporator 3 to the absorber 4 where it istaken into solution with absorbent (lithium bromide). A flow ofrefrigerant vapour is maintained by a boiling process within evaporator3, thus creating the necessary refrigeration effect. The absorptionprocess is exothermic and, therefore, the absorber 4 requires constantcooling to maintain its temperature. As refrigerant enters solution withthe absorbent, its ability to absorb water vapour decreases. To maintainthe strength of the absorbent a quantity of the solution is continuouslypumped, at high pressure, to generator 1 where it is heated causing therefrigerant water to be driven out of the solution which is thenreturned to absorber 4, via a pressure regulator valve 5. The highpressure refrigerant vapour flows from generator 1 to condenser 2 whereit is liquefied and returned, via an expansion valve 6 to evaporator 3,thus completing the cycle. A solution heat exchanger 7 may be added topre-heat the solution leaving the absorber using he hot solutionreturning from generator 1. Thus generator 1 input is reduced, and thesystem performance is improved.

In contrast, in FIG. 2 there is illustrated an absorptionheat/refrigerator system in accordance with the invention. There isprovided an ejector 8 located downstream of evaporator 3 and generator1, but upstream of condenser 2. Refrigerant vapour issuing fromgenerator 1 drives ejector 8 which in turn entrains refrigerant vapourfrom evaporator 3. Moreover, as described with reference to FIG. 1,absorbent in absorber 4 also entrains refrigerant vapour from evaporator3. Thus in the system of the invention two means 8 and 4 are providedfor entraining refrigerant vapour from evaporator 3 thus enhancing theperformance of the system. However, refrigerant vapour leavingevaporator 3 and passing through ejector 8 is delivered to condenser 2.This means that the processing burden on absorber 4 is significantlyreduced since refrigerant vapour passing through ejector 8 is compressedand so condenses within condenser 2.

The burden or load on absorber 4 is significantly reduced and, as aresult of this, the size and complexity of absorber 4 can be reduced bya half to two-thirds of that normally found in a conventional andcomparable system.

It is also of note that all of the refrigerant flowing through thesystem of the invention passes directly through the evaporator and istherefore used for heat exchange with the surrounding environment.

The amount of vapour withdrawn from the evaporator by the ejector willdetermine both the performance of the system and the efficiency ofcooling of the system. The greater the amount of vapour withdrawn thegreater the cooling performance.

FIG. 3 shows an ejector-absorption system in accordance with theinvention which includes a separator 9. Separator 9 is provided tocontrol the re-charging or dehydration of the absorbent solution flowingthrough the system. In the system shown in FIG. 2, re-charging of theabsorbent is, to a large extent, determined by the refrigerant vapourpassing from the generator 1 through ejector 2. Thus the rate of flow ofrefrigerant vapour through ejector 8 has a significant controllingeffect on the re-changing of the absorbent. In contrast, in the systemshown in FIG. 3, a separator 9 is provided so that absorbent which haspassed through generator 1 and is returning to absorber 4 can be furtherre-charged in separator 9 and the refrigerant vapour that is produced ispassed to condenser 2 via feed-line 10. Re-charging in separator 9 maybe brought about by conventional techniques such as expansion. Theprovision of separator 9 will depend upon the nature of the absorbent tobe used and it may be that with certain absorbents such as separator isbeneficial in controlling the way the system operates.

FIG. 4 shows an ejector-absorption system in accordance with theinvention which further includes an ejector economiser 11. Economiser 11is provided downstream of ejector 8 and upstream of condenser 2.Economiser 11 is used to heat absorbent solution prior to its passagethough generator 1. Thus absorbent leaving absorber 4 travels alongfeed-line 12 which feed-line diverges at point X so that a parallel flowis created through feed-line 13. Line 13 travels through economiser 11and then to generator 1 via feed-line 13a. Moreover, refrigerant vapourwhich has passed through generator 1 and ejector 8 also passes througheconomiser 11. Thus heat from this refrigerant vapour is used to heatabsorbent flowing through feed-line 13. Absorbent passing via feed-line13a to generator 1 is thus pre-heated prior to entering generator 1.This increases the efficiency of the system.

In addition, it can also be seen that refrigerant vapour entrained fromevaporator 3 and passing through ejector 8 also passes througheconomiser 11.

Thus, refrigerant vapour drawn from generator 1 and evaporator 3 is usedto pre-heat absorbent passing through feed-line 13. This arrangementreduces the load on the generator and provides for reduced external heattransfer at the condenser. This means that the size/capacity of thecondenser can be reduced.

It is of note that application of the invention to heaters and boilersfalls within the scope of the invention and further the invention isalso applicable to exploitation in the chemical and process industries,the main thrust of the invention involving the provision of an ejectorbetween an evaporator and a condenser so as to alter the performance ofthe system.

We claim:
 1. A heat pump and refrigeration system comprising:a generatorfor producing heat to power the system; a condenser for rejecting heatfrom the system; an evaporator for effecting heat exchange with anenvironment; an absorber for extracting refrigerant vapour from theevaporator; and an ejector for extracting refrigerant vapour from saidevaporator, wherein all of the refrigerant in the system passes throughsaid evaporator in each cycle and wherein said ejector is positioneddownstream of said evaporator and upstream of said condenser so thatrefrigerant vapour extracted from said evaporator by said ejector passesthrough said ejector before being delivered to said condenser.
 2. Asystem according to claim 1 wherein the ejector is further positioneddownstream of the generator so that refrigerant vapour issuing from thegenerator passes through the ejector and so brings about entrainment ofrefrigerant vapour from the evaporator.
 3. A system according to claim 1wherein a circuit is created so that all refrigerant vapour passesthrough the evaporator and then a fraction of that vapour leaves theevaporator and passes to the ejector and the remaining fraction leavesthe evaporator and passes to the absorber.
 4. A system according toclaim 1 wherein there is further provided a separator positioned betweenthe generator and the absorber so that absorbent returning from thegenerator to the absorber passes through the separator and so releasesrefrigerant vapour.
 5. A system according to claim 4 wherein saidseparator is in fluid connection with said condenser so that saidrefrigerant vapour yield by the absorbent passes from the separator tothe condenser.
 6. A system according to claim 1 wherein there is furtherprovided an ejector-economiser which is positioned downstream of theejector and which is provided with a feed-line which draws absorbentfrom the absorber to the economiser and then delivers the absorbent,after passage through the economiser, to the generator.
 7. A systemaccording to claim 6 wherein the feed-line is provided downstream of theabsorber so that absorbent leaving the absorber on its way to thegenerator is in part diverted so as to pass through the economiser.